The C-terminal region of Rad52 is essential for Rad52 nuclear and nucleolar localization, and accumulation at DNA damage sites immediately after irradiation

Phase separation in DNA double-strand break response

DNA double-strand break (DSB) is the most dangerous type of DNA damage, which may lead to cell death or oncogenic mutations. Homologous recombination (HR) and nonhomologous end-joining (NHEJ) are two typical DSB repair mechanisms. Recently, many studies have revealed that liquid-liquid phase separation (LLPS) plays a pivotal role in DSB repair and response. Through LLPS, the crucial biomolecules are quickly recruited to damaged sites with a high concentration to ensure DNA repair is conducted quickly and efficiently, which facilitates DSB repair factors activating downstream proteins or transmitting signals. In addition, the dysregulation of the DSB repair factor's phase separation has been reported to promote the development of a variety of diseases. This review not only provides a comprehensive overview of the emerging roles of LLPS in the repair of DSB but also sheds light on the regulatory patterns of phase separation in relation to the DNA damage response (DDR).

Mechanism of single-stranded DNA annealing by RAD52-RPA complex

RAD52 is important for the repair of DNA double-stranded breaks1,2, mitotic DNA synthesis3-5 and alternative telomere length maintenance6,7. Central to these functions, RAD52 promotes the annealing of complementary single-stranded DNA (ssDNA)8,9 and provides an alternative to BRCA2/RAD51-dependent homologous recombination repair10. Inactivation of RAD52 in homologous-recombination-deficient BRCA1- or BRCA2-defective cells is synthetically lethal11,12, and aberrant expression of RAD52 is associated with poor cancer prognosis13,14. As a consequence, RAD52 is an attractive therapeutic target against homologous-recombination-deficient breast, ovarian and prostate cancers15-17. Here we describe the structure of RAD52 and define the mechanism of annealing. As reported previously18-20, RAD52 forms undecameric (11-subunit) ring structures, but these rings do not represent the active form of the enzyme. Instead, cryo-electron microscopy and biochemical analyses revealed that ssDNA annealing is driven by RAD52 open rings in association with replication protein-A (RPA). Atomic models of the RAD52-ssDNA complex show that ssDNA sits in a positively charged channel around the ring. Annealing is driven by the RAD52 N-terminal domains, whereas the C-terminal regions modulate the open-ring conformation and RPA interaction. RPA associates with RAD52 at the site of ring opening with critical interactions occurring between the RPA-interacting domain of RAD52 and the winged helix domain of RPA2. Our studies provide structural snapshots throughout the annealing process and define the molecular mechanism of ssDNA annealing by the RAD52-RPA complex.

Exome sequencing identified rare recurrent copy number variants and hereditary breast cancer susceptibility

Copy number variants (CNVs) are a major source of genetic variation and can disrupt genes or affect gene dosage. They are known to be causal or underlie predisposition to various diseases. However, the role of CNVs in inherited breast cancer susceptibility has not been thoroughly investigated. To address this, we performed whole-exome sequencing based analysis of rare CNVs in 98 high-risk Northern Finnish breast cancer cases. After filtering, selected candidate alleles were validated and characterized with a combination of orthogonal methods, including PCR-based approaches, optical genome mapping and long-read sequencing. This revealed three recurrent alterations: a 31 kb deletion co-occurring with a retrotransposon insertion (delins) in RAD52, a 13.4 kb deletion in HSD17B14 and a 64 kb partial duplication of RAD51C. Notably, all these genes encode proteins involved in pathways previously identified as essential for breast cancer development. Variants were genotyped in geographically matched cases and controls (altogether 278 hereditary and 1983 unselected breast cancer cases, and 1229 controls). The RAD52 delins and HSD17B14 deletion both showed significant enrichment among cases with indications of hereditary disease susceptibility. RAD52 delins was identified in 7/278 cases (2.5%, P = 0.034, OR = 2.86, 95% CI = 1.10-7.45) and HSD17B14 deletion in 8/278 cases (2.9%, P = 0.014, OR = 3.28, 95% CI = 1.31-8.23), the frequency of both variants in the controls being 11/1229 (0.9%). This suggests a role for RAD52 and HSD17B14 in hereditary breast cancer susceptibility. The RAD51C duplication was very rare, identified only in 2/278 of hereditary cases and 2/1229 controls (P = 0.157, OR = 4.45, 95% CI = 0.62-31.70). The identification of recurrent CNVs in these genes, and especially the relatively high frequency of RAD52 and HSD17B14 alterations in the Finnish population, highlights the importance of studying CNVs alongside single nucleotide variants when searching for genetic factors underlying hereditary disease predisposition.

A moving target for drug discovery: Structure activity relationship and many genome (de)stabilizing functions of the RAD52 protein

BRCA-ness phenotype, a signature of many breast and ovarian cancers, manifests as deficiency in homologous recombination, and as defects in protection and repair of damaged DNA replication forks. A dependence of such cancers on DNA repair factors less important for survival of BRCA-proficient cells, offers opportunities for development of novel chemotherapeutic interventions. The first drugs targeting BRCA-deficient cancers, poly-ADP-ribose polymerase (PARP) inhibitors have been approved for the treatment of advanced, chemotherapy resistant cancers in patients with BRCA1/2 germline mutations. Nine additional proteins that can be targeted to selectively kill BRCA-deficient cancer cells have been identified. Among them, a DNA repair protein RAD52 is an especially attractive target due to general tolerance of the RAD52 loss of function, and protective role of an inactivating mutation. Yet, the effective pharmacological inhibitors of RAD52 have not been forthcoming. In this review, we discuss advances in the state of our knowledge of the RAD52 structure, activities and cellular functions, with a specific focus on the features that make RAD52 an attractive, but difficult drug target.

Phospho-Ku70 induced by DNA damage interacts with RNA Pol II and promotes the formation of phospho-53BP1 foci to ensure optimal cNHEJ

Canonical non-homologous end-joining (cNHEJ) is the prominent mammalian DNA double-strand breaks (DSBs) repair pathway operative throughout the cell cycle. Phosphorylation of Ku70 at ser27-ser33 (pKu70) is induced by DNA DSBs and has been shown to regulate cNHEJ activity, but the underlying mechanism remained unknown. Here, we established that following DNA damage induction, Ku70 moves from nucleoli to the sites of damage, and once linked to DNA, it is phosphorylated. Notably, the novel emanating functions of pKu70 are evidenced through the recruitment of RNA Pol II and concomitant formation of phospho-53BP1 foci. Phosphorylation is also a prerequisite for the dynamic release of Ku70 from the repair complex through neddylation-dependent ubiquitylation. Although the non-phosphorylable ala-Ku70 form does not compromise the formation of the NHEJ core complex per se, cells expressing this form displayed constitutive and stress-inducible chromosomal instability. Consistently, upon targeted induction of DSBs by the I-SceI meganuclease into an intrachromosomal reporter substrate, cells expressing pKu70, rather than ala-Ku70, are protected against the joining of distal DNA ends. Collectively, our results underpin the essential role of pKu70 in the orchestration of DNA repair execution in living cells and substantiated the way it paves the maintenance of genome stability.

The function of RAD52 N-terminal domain is essential for viability of BRCA-deficient cells

RAD52 is a member of the homologous recombination pathway that is important for survival of BRCA-deficient cells. Inhibition of RAD52 leads to lethality in BRCA-deficient cells. However, the exact mechanism of how RAD52 contributes to viability of BRCA-deficient cells remains unknown. Two major activities of RAD52 were previously identified: DNA or RNA pairing, which includes DNA/RNA annealing and strand exchange, and mediator, which is to assist RAD51 loading on RPA-covered ssDNA. Here, we report that the N-terminal domain (NTD) of RAD52 devoid of the potential mediator function is essential for maintaining viability of BRCA-deficient cells owing to its ability to promote DNA/RNA pairing. We show that RAD52 NTD forms nuclear foci upon DNA damage in BRCA-deficient human cells and promotes DNA double-strand break repair through two pathways: homology-directed repair (HDR) and single-strand annealing (SSA). Furthermore, we show that mutations in the RAD52 NTD that disrupt these activities fail to maintain viability of BRCA-deficient cells.

Variants of the human RAD52 gene confer defects in ionizing radiation resistance and homologous recombination repair in budding yeast

RAD52 is a structurally and functionally conserved component of the DNA double-strand break (DSB) repair apparatus from budding yeast to humans. We recently showed that expressing the human gene, HsRAD52 in rad52 mutant budding yeast cells can suppress both their ionizing radiation (IR) sensitivity and homologous recombination repair (HRR) defects. Intriguingly, we observed that HsRAD52 supports DSB repair by a mechanism of HRR that conserves genome structure and is independent of the canonical HR machinery. In this study we report that naturally occurring variants of HsRAD52, one of which suppresses the pathogenicity of BRCA2 mutations, were unable to suppress the IR sensitivity and HRR defects of rad52 mutant yeast cells, but fully suppressed a defect in DSB repair by single-strand annealing (SSA). This failure to suppress both IR sensitivity and the HRR defect correlated with an inability of HsRAD52 protein to associate with and drive an interaction between genomic sequences during DSB repair by HRR. These results suggest that HsRAD52 supports multiple, distinct DSB repair apparatuses in budding yeast cells and help further define its mechanism of action in HRR. They also imply that disruption of HsRAD52-dependent HRR in BRCA2-defective human cells may contribute to protection against tumorigenesis and provide a target for killing BRCA2-defective cancers.

The RAD52 S346X variant reduces risk of developing breast cancer in carriers of pathogenic germline BRCA2 mutations

Women who carry pathogenic mutations in BRCA1 and BRCA2 have a lifetime risk of developing breast cancer of up to 80%. However, risk estimates vary in part due to genetic modifiers. We investigated the association of the RAD52 S346X variant as a modifier of the risk of developing breast and ovarian cancers in BRCA1 and BRCA2 mutation carriers from the Consortium of Investigators of Modifiers of BRCA1/2. The RAD52 S346X allele was associated with a reduced risk of developing breast cancer in BRCA2 carriers [per‐allele hazard ratio (HR) = 0.69, 95% confidence interval (CI) 0.56–0.86; P = 0.0008] and to a lesser extent in BRCA1 carriers (per‐allele HR = 0.78, 95% CI 0.64–0.97, P = 0.02). We examined how this variant affected DNA repair. Using a reporter system that measures repair of DNA double‐strand breaks (DSBs) by single‐strand annealing (SSA), expression of hRAD52 suppressed the loss of this repair in Rad52^−/− mouse embryonic stem cells. When hRAD52 S346X was expressed in these cells, there was a significantly reduced frequency of SSA. Interestingly, expression of hRAD52 S346X also reduced the stimulation of SSA observed upon depletion of BRCA2, demonstrating the reciprocal roles for RAD52 and BRCA2 in the control of DSB repair by SSA. From an immunofluorescence analysis, we observed little nuclear localization of the mutant protein as compared to the wild‐type; it is likely that the reduced nuclear levels of RAD52 S346X explain the diminished DSB repair by SSA. Altogether, we identified a genetic modifier that protects against breast cancer in women who carry pathogenic mutations in BRCA2 (P = 0.0008) and to a lesser extent BRCA1 (P = 0.02). This RAD52 mutation causes a reduction in DSB repair by SSA, suggesting that defects in RAD52‐dependent DSB repair are linked to reduced tumor risk in BRCA2‐mutation carriers.

Rad52 competes with Ku70/Ku86 for binding to S-region DSB ends to modulate antibody class-switch DNA recombination

Antibody class-switch DNA recombination (CSR) is initiated by AID-introduced DSBs in the switch (S) regions targeted for recombination, as effected by Ku70/Ku86-mediated NHEJ. Ku-deficient B cells, however, undergo (reduced) CSR through an alternative(A)-NHEJ pathway, which introduces microhomologies in S-S junctions. As microhomology-mediated end-joining requires annealing of single-strand DNA ends, we addressed the contribution of single-strand annealing factors HR Rad52 and translesion DNA polymerase θ to CSR. Compared with their Rad52+/+ counterparts, which display normal CSR, Rad52-/- B cells show increased CSR, fewer intra-Sμ region recombinations, no/minimal microhomologies in S-S junctions, decreased c-Myc/IgH translocations and increased Ku70/Ku86 recruitment to S-region DSB ends. Rad52 competes with Ku70/Ku86 for binding to S-region DSB ends. It also facilitates a Ku-independent DSB repair, which favours intra-S region recombination and mediates, particularly in Ku absence, inter-S-S recombination, as emphasized by the significantly greater CSR reduction in Rad52-/- versus Rad52+/+ B cells on Ku86 knockdown.

Cloning, localization and focus formation at DNA damage sites of canine XRCC4

Various chemotherapies and radiation therapies are useful for killing cancer cells mainly by inducing DNA double-strand breaks (DSBs). Uncovering the molecular mechanisms of DSB repair processes is crucial for developing next-generation radiotherapies and chemotherapeutics for human and animal cancers. XRCC4 plays a critical role in Ku-dependent nonhomologous DNA-end joining (NHEJ) in human cells, and is one of the core NHEJ factors. The localization of core NHEJ factors, such as human Ku70 and Ku80, might play a crucial role in regulating NHEJ activity. Recently, companion animals, such as canines, have been proposed to be a good model in many aspects of cancer research. However, the localization and regulation mechanisms of core NHEJ factors in canine cells have not been elucidated. Here, we show that the expression and subcellular localization of canine XRCC4 changes dynamically during the cell cycle. Furthermore, EYFP-canine XRCC4 accumulates quickly at laser-microirradiated DSB sites. The structure of a putative human XRCC4 nuclear localization signal (NLS) is highly conserved in canine, chimpanzee and mouse XRCC4. However, the amino acid residue corresponding to the human XRCC4 K210, thought to be important for nuclear localization, is not conserved in canine XRCC4. Our findings might be useful for the study of the molecular mechanisms of Ku-dependent NHEJ in canine cells and the development of new radiosensitizers that target XRCC4.

Reappearance from Obscurity: Mammalian Rad52 in Homologous Recombination

Homologous recombination (HR) plays an important role in maintaining genomic integrity. It is responsible for repair of the most harmful DNA lesions, DNA double-strand breaks and inter-strand DNA cross-links. HR function is also essential for proper segregation of homologous chromosomes in meiosis, maintenance of telomeres, and resolving stalled replication forks. Defects in HR often lead to genetic diseases and cancer. Rad52 is one of the key HR proteins, which is evolutionarily conserved from yeast to humans. In yeast, Rad52 is important for most HR events; Rad52 mutations disrupt repair of DNA double-strand breaks and targeted DNA integration. Surprisingly, in mammals, Rad52 knockouts showed no significant DNA repair or recombination phenotype. However, recent work demonstrated that mutations in human RAD52 are synthetically lethal with mutations in several other HR proteins including BRCA1 and BRCA2. These new findings indicate an important backup role for Rad52, which complements the main HR mechanism in mammals. In this review, we focus on the Rad52 activities and functions in HR and the possibility of using human RAD52 as therapeutic target in BRCA1 and BRCA2-deficient familial breast cancer and ovarian cancer.

Nuclear localization of mouse Ku70 in interphase cells and focus formation of mouse Ku70 at DNA damage sites immediately after irradiation

To elucidate the mechanisms of DNA repair pathway is critical for developing next-generation radiotherapies and chemotherapeutic drugs for cancer. Ionizing radiation and many chemotherapeutic drugs kill tumor cells mainly by inducing DNA double-strand breaks (DSBs). The classical nonhomologous DNA-end joining (NHEJ) (C-NHEJ) pathway repairs a predominant fraction of DSBs in mammalian cells. The C-NHEJ pathway appears to start with the binding of Ku (heterodimer of Ku70 and Ku80) to DNA break ends. Therefore, recruitment of Ku to DSB sites might play a critical role in regulating NHEJ activity. Indeed, human Ku70 and Ku80 localize in the nuclei and accumulate at microirradiated DSB sites. However, the localization and regulation mechanisms of Ku70 and Ku80 homologues in animal models, such as mice and other species, have not been elucidated in detail, particularly in cells immediately after microirradiation. Here, we show that EYFP-tagged mouse Ku70 localizes in the interphase nuclei of mouse fibroblasts and epithelial cells. Furthermore, our findings indicate that EYFP-mouse Ku70 accumulates with its heterodimeric partner Ku80 immediately at laser-microirradiated DSB sites. We also confirmed that the structure of Ku70 nuclear localization signal (NLS) is highly conserved among various rodent species, such as the mouse, rat, degu and ground squirrel, supporting the idea that NLS is important for the regulation of rodent Ku70 function. Collectively, these results suggest that the mechanisms of regulating the localization and accumulation of Ku70 at DSBs might be well conserved between the mouse and human species.

Dynamic changes in subcellular localization of cattle XLF during cell cycle, and focus formation of cattle XLF at DNA damage sites immediately after irradiation

Clinically, many chemotherapeutics and ionizing radiation (IR) have been applied for the treatment of various types of human and animal malignancies. These treatments kill tumor cells by causing DNA double-strand breaks (DSBs). Core factors of classical nonhomologous DNA-end joining (C-NHEJ) play a vital role in DSB repair. Thus, it is indispensable to clarify the mechanisms of C-NHEJ in order to develop next-generation chemotherapeutics for cancer. The XRCC4-like factor (XLF; also called Cernunnos or NHEJ1) is the lastly identified core NHEJ factor. The localization of core NHEJ factors might play a critical role in regulating NHEJ activity. The localization and function of XLF have not been elucidated in animal species other than mice and humans. Domestic cattle (Bos taurus) are the most common and vital domestic animals in many countries. Here, we show that the localization of cattle XLF changes dynamically during the cell cycle. Furthermore, EYFP-cattle XLF accumulates quickly at microirradiated sites and colocalizes with the DSB marker γH2AX. Moreover, nuclear localization and accumulation of cattle XLF at DSB sites are dependent on 12 amino acids (288-299) of the C-terminal region of XLF (XLF CTR). Furthermore, basic amino acids on the XLF CTR are highly conserved among domestic animals including cattle, goat and horses, suggesting that the CTR is essential for the function of XLF in domestic animals. These findings might be useful to develop the molecular-targeting therapeutic drug taking XLF as a target molecule for human and domestic animals.